Currently, resins are used as replacements for dental tissue after diseased tissue has been excised by the use of dental burs and excavators. Typical uses are restoration of teeth, cementing crowns and inlays and fixing orthodontic devices in places. They may also be used as bases and liners in conventional cavities. Further hydrophilic resin mixtures may be used to mechanically interlock with the surface of the freshly cut dentine to bind the resin to the tooth in an attempt to achieve a hermetic seal.
These resins are usually a composite of filler and a polymerisable resin based on one or more methacrylates or acrylates. The base resins used are often of high viscosity and are hydrophobic. Long chain materials are thought to reduce the shrinkage on polymerisation. The mechanical properties of the resin alone are inadequate for the restoration of conventional cavities and the filler is added to enhance the mechanical properties and impart radiopacity The filler is generally an inert glass or silica and takes no part in the setting reaction. The filler is normally coated with a silane-coupling agent to enhance the union between the glass and the resin. This coupling agent is normally a difunctional vinyl silane. Setting is achieved by free radical polymerisation of the monomer component. This may be achieved in a number of ways and is dependent on the presentation of the material. The most common presentation is a paste, which contains an α diketone and an amine reducing agent. The diketone is excited by a high intensity light of a wavelength generally between 450–480 m, and the polymerisation reaction occurs. Alternatively, the materials may be presented as two components, generally pastes. The polymerisation reaction is initiated by mixing the two pastes together. Each paste will contain either benzoyl peroxide or a tertiary amine. The combination of these two chemicals results in polymerisation of the resin.
These resins have good aesthetic properties and the addition of filler provides adequate mechanical properties for some uses. However, the nature of the setting reaction and the polymerisation shrinkage and exotherm presents problems associated with marginal leakage around the restoration. This can cause further problems as bacteria can track between the restoration and the tooth structure. This will lead to recurrent infection and further caries. Some materials, which use resin technology also, contain polyacids, which react with the filler and form a polysalt matrix. These are known as resin modified glass ionomer cements. These have the advantage that the polysalt matrix contains fluoride ions, which may be released and diffuse out from the restorative into the surrounding tooth structure and oral cavity. The presence of this polysalt matrix weakens the set material and results in a restoration, which is not as strong as ordinary resin composite. These materials are not suitable for load bearing restorations and form a stiff paste prior to setting. Further developments have utilised both dimethacrylate resins but also difunctional monomers where carboxylate groups are present, which react with water after the restoration has been placed. The main purpose of this is to provide some fluoride release while maintaining the mechanical properties of the resin composites. These still suffer from the problems of polymerisation shrinkage and exotherm.
Resin composites and their hybrids rely for retention on mechanical preparation of the enamel and for retention to dentine there is a secondary material applied as a dentine adhesive prior to the application of the composite. The materials are in the form of a paste in order to facilitate the retention of the material in a wide cavity. Alternative materials for restoration of teeth are the glass ionomer cements, which set primarily by an acid base reaction between the fluoro-alumino-silicate glass and the polyacid such as poly acrylic acid. Again, these materials are required to be a stiff paste to aid insertion and retention within a conventional cavity. Their main benefit is adhesion to both enamel and dentine and also the sustained release of fluoride over periods in excess of four years.